The present invention relates to a rotary blower and more particularly, to a torsion damping mechanism for reducing audible noise from the timing gears in a rotary blower driven by an internal combustion engine.
It should be understood by those skilled in the art that the present invention is not limited to a Roots-type blower, but could be used just as advantageously in a screw compressor type of blower. A Roots-type blower transfers volumes of air from the inlet port to the outlet port, whereas a screw compressor actually achieves internal compression of the air before delivering it to the outlet port. However, for purposes of the present invention, what is most important is that the blower include a pair of rotors which must be timed in relationship to each other, and therefore, are driven by meshed timing gears which are potentially subject to conditions such as gear rattle and bounce as described above.
Rotary blowers of the type to which the present invention relates are also referred to as "supercharges" because they effectively super charge the intake of the engine. Typically, the pulley and belt drive arrangement for a Roots blower supercharger is sized such that, at any given engine speed, the amount of air being transferred into the intake manifold is greater than the instantaneous displacement of the engine, thus increasing the air pressure within the intake manifold, and increasing the power density of the engine.
Rotary blowers of either the Roots type or the screw compressor type, are characterized by the potential to generate noise. For example, Roots-type blower noise may be classified as either of two types. The first is solid borne noise caused by rotation of timing gears and rotor shaft bearings subjected to fluctuating loads (the firing pulses of the engine), and the second is fluid borne noise caused by fluid flow characteristics, such as rapid changes in fluid (air) velocity. The present invention is concerned primarily with the solid borne noise caused by the meshing of the timing gears. More particularly, the present invention is concerned with minimizing the "bounce" of the timing gears during times of relatively low speed operation, when the blower rotors are not "under load". Thus, it is important to be able to isolate the fluctuating input to the supercharger from the timing gears. The noise which may be produced by the meshed teeth of the timing gears during unloaded (non-supercharging) low speed operation is also referred to as "gear rattle".
An example of a prior art torsion damping mechanism for a supercharger is illustrated and described in U.S. Pat. No. 4,844,044, assigned to the assignee of the present invention, and incorporated herein by reference. Although the device of the incorporated patent has been generally satisfactory in terms of operational performance, the number of parts required, and the nature of those parts, and the requirement for two different spring members, has in some cases made the total manufacturing and assembly cost of the torsion damping mechanism exceed what is commercially feasible for the particular vehicle application.
Inherent in the design of the torsion damping mechanism of the above-incorporated patent is a very limited amount of travel in the positive torque direction. For example, in a damping mechanism sold commercially by the assignee of the present invention, the maximum travel was in the range of about 10 to about 15 degrees. The only way to adapt ("tune") a particular damping mechanism to a different engine application (i.e., a different input impulse loading) is to replace the spring with one having a different spring rate. However, in many cases the result would be a spring which would be too stiff for the particular engine application.
Typically, the known prior art torsion damping mechanisms utilized between the input shaft and the timing gears of vehicle engine superchargers have operated in either an isolation (damping) mode, such as when torque is being transmitted through a spring, or in a direct drive mode, when the damping mechanism effectively performs like a solid mechanical member. Unfortunately, in most of the conventional torsion damping mechanisms, the transition between the isolation mode and the direct drive mode has been sudden, rather than gradual. An abrupt transition between operating modes can generate noise, such as from the impact of engagement of various elements of the torsion damping mechanism.